"Hiroshima" is fantastic! Absolutely journalism at its finest, some of the best ever done by anyone. It may not have been the first reporting in the popular style (the great war correspondent Ernie Pyle did it in a popular style, too, though he wrote shorter pieces). But "Hiroshima" was probably the most acclaimed early example of long-form reporting that reads like a novel. Most of Hemingway's journalism came before it, unless you count "A Movable Feast" (which may have preceded but is a memoir that is apparently slightly fictionalized--EH is a bit cagey about that in the book).